This application claims the priority of German patent application No. 196 34 504.9, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a light-metal-part blank which is to be cast into another light-metal casting, and has a roughness of more than 20 .mu.m on its outer surface, and also to a method for producing the blank in which method the surface of a blank is blasted with a directed jet of particles which consist of a hard material and are carried along in a flowing gas.
DE 44 38 550 A1 describes the casting of a cylinder liner into a crankcase. Casting separately manufactured cylinder liners into light-metal crankcases has successfully optimized the running properties of the reciprocating piston in the cylinder liner, irrespective of the material of the crankcase. Problems with casting the cylinder liners into the light-metal crankcase arise, however, due to the inadequacy of the bonding of the outside of the liner with the crankcase material. When the engine is running, materially imperfect bonding can cause the emission of waste heat from the reciprocating-piston engine to be impeded. In particularly unfavorable instances, this emission can even lead to a loosening of the cylinder liner in the crankcase. As regards other parts to be cast in, for example forged rotor recesses in a cast piston, good bonding is indispensable, for strength reasons alone.
DE 43 28 619 C2 discusses problems involved in good material bonding of the light-metal components during casting in, in particular in the instance of a cylinder liner to be cast in. An objective is a pore-free material union between the outside of the liner and the case material by controlled preheating of the cylinder liner. The cylinder-liner blank preheated to a specific temperature, for example 450.degree. C., and introduced into the casting mold has its surface melted (incipiently) by the inflowing melt of the case material, and an intimate bond with the case material is thereby made. A high melt flow directed parallel to the contact surface further assists this effect, not only by bringing about increased incipient melting as a result of a better heat exchange, but also by washing off the oxide skin, which is always present, from the contact side of the liner.
Such an intensive relative flow of the melt can be ensured by various measures. The above-mentioned publication mentions, for example, a choice and distribution of the gates, an agitation of the melt or even an induction of electrical eddy currents which cause fluid flows in the melt. A disadvantage of this method, however, is that the liner blanks preheated to temperatures which bring about reliable incipient melting are difficult to handle, especially during the casting of multi-cylinder crankcases. With the gradual introduction of the individual preheated liners into the casting die, either different liner temperatures have to be allowed for, due to cooling, during the casting operation or heating elements have to be provided in the casting die so that the liner blanks already introduced are kept hot, thus making the casting die more complicated and adversely affecting the dissipation of heat from the solidifying cast workpiece.
In any event, a preheating furnace must be installed, and this installation incurs further investment costs, above all, regular power-supply costs. Moreover, the high preheating temperatures may lead to undesirable structural changes in the material of the cylinder liner which can adversely influence the liner's running properties. Tribologically relevant structural changes are obtained if the liner blank, while being cast in, is melted down nearly into the region of the running surface.
A machining oversize of at least about 1 mm provided on the inside of the liner blank must be taken into account. In order, therefore, to prevent the liner blank from actually melting through at all locations, a correspondingly thick-walled blank has to be provided. For reasons of the smallest possible cylinder spacing, however, the cylinder liner should be as thin-walled as possible. If, for whatever reason, the liner is not sufficiently preheated, i.e. by way of precaution or through carelessness, then, at least in die casting, only very short periods of time are available for filling the mold and until solidification commences. Consequently, the aforementioned incipient-melting measures cannot take effect, or can take effect only very incompletely, in the short time periods available.
An object of the present invention is to improve the blank of a light-metal structural part to be cast in, and the corresponding production method. Thereby, the blanks, while being cast in, make an intimate material union over a wide area with the cast material of the cast-round part, even without preheating.
This and other objects have been achieved, according to the present invention, by providing a light-metal-part blank which is to be cast into another light-metal casting and has a roughness of more than 20 .mu.m on its outer surface, which is to be surrounded by the material of the light-metal casting, the topography of this surface being formed by tapering, approximately pyramid-like or lancet-like protruding material scabs or material accumulations, which merge directly at their base into the basic structure of the blank.
Likewise, the improved method achieves the aforementioned object by in which method first of all a blank is produced and machined to the desired shape and desired size and, subsequently, the outer surface of the blank, which surface is to be surrounded by the material of the casting, is blasted with a directed jet of particles which consist of a hard material and are carried along in a flowing gas.
It is important that the outer contact surface of the blank has a topography with a multiplicity of tapering material elevations, for example of pyramid-like or lancet-like form, which merge, undisturbed, at their base, over a wide area, into the basic material of the blank. Notwithstanding the existing oxide skin, the tips of the multiplicity of small pyramid-like or lancet-like protruding material scabs or material accumulations on the contact side of the blank immediately begin to melt, in their tip region when they come into contact with the melt of the cast-round part. This results from the small contact zone having sufficiently high heat energy supplied by contact with the melt, with heat dissipation into the depth of the material being initially still low. Thus, a sufficient energy density is locally available in order to overcome the barrier of the oxide skin locally.
The incipient melting which has been initiated spreads very quickly in the near-surface layer on the contact side of the blank. The pyramid-like or lancet-like protruding material scabs or material accumulations thus constitute initiating locations for the incipient-melting operation. Because of the rapid progress of an incipient-melting operation once begun and of dense covering of the contact side by such initiating locations, the locations where incipient melting has begun very quickly coalesce into a continuous near-surface incipient-melting zone. The incipient melting therefore spreads quickly over the surface area, but penetrates only relatively little into the depth of the blank wall. Thereby, the structure remains unaffected on the opposite side of the wall of the blank, for example on the piston running side.
The following are among the numerous and widely differing advantages can be achieved with the present invention;
preheating of the cast-in part, in particular the liner blank to be cast in, is eliminated along with the associated investment and operating costs and handling problems; PA1 roughening the outer or contact surface of the cast-in part also achieves the effect of cleaning, which is necessary in any case, so that separate cleaning is unnecessary; the outlay in terms of investment costs and regular costs for roughening is approximately comparable to that for cleaning, so that roughening requires virtually no extra outlay; PA1 in the case of liner blanks to be cast in, tribologically relevant structural changes on the running side of the liner blank can be avoided with a high degree of process reliability; PA1 allowing the cast-in part to have smaller wall thicknesses; at the very least, smaller wall thicknesses can be controlled with greater process reliability than in a casting-in operation with preheating of the casting; PA1 providing smaller cylinder wall thicknesses to allow smaller cylinder spacings and therefore, with the piston capacity remaining the same, shorter, lighter and more cost-effective engines; this, in turn allows smaller engine spaces in the motor vehicle and, due to the mass involved, lower fuel consumption for the motor vehicle driven thereby; PA1 in comparison with the casting in of non-roughened cast-in parts, achieving a better metallurgical bond which is largely of uniformly high quality over the extent of the contact surface between the cast-in part and the cast-round part; PA1 as a result, where cylinder liners are concerned, as measurements have shown, higher manufacturing accuracy, in particular less manufacturing related cylinder warping, can be achieved, because a cylinder liner which has good bonding to the crankcase allows the crankcase to be more rigid than a liner essentially only positively surrounded; PA1 due to the better metallurgical bonding of the liner to the crankcase material, a higher rigidity is achieved along with a cylinder wall which is uniform in the circumferential and axial directions (i.e. homogeneous), and., when the cylinder head is being assembled, with a gasket interposed, less assembly-related cylinder warping; PA1 by virtue of the high-strength material bonding of the cylinder liner in the crankcase, there is no need for retaining collars on the end faces of the liner; the liner is thereby configured particularly simply from a manufacturing point of view and can thus be produced cost-effectively; PA1 as regards cylinder liners, due to the better metallurgical bonding of the liner to the case material, better heat transmission which is more uniform over the surface area, a more uniform temperature profile of the cylinder liner in the circumferential and axial directions and less thermally related cylinder warping can be achieved when the engine is running; PA1 moreover, the temperature level of the well bonded-in cylinder liner as a whole is lower than in cylinder liners which are cast in without being roughened; this has a favorable effect on the oil evaporation rate when the engine is running and therefore on the oil consumption and on the exhaust gas content of hydrocarbons produced by the lubricating oil; PA1 higher manufacturing-related dimensional accuracy, less assembly-related cylinder warping and less operation-related thermal warping of the cylinder liners, in turn, achieve a smaller piston clearance which has a favorable effect on the exhaust gas content of hydrocarbons produced by the fuel; PA1 the high dimensional accuracy of the running surface reduces piston vibration and thus results in smoother engine operation; and PA1 the high dimensional accuracy of the running surface also results in a better sealing effect of the piston rings and therefore lower blow-through losses and a lower oil consumption (i.e., higher efficiency), lower fuel consumption and lower emissions, particularly of oil-produced hydrocarbons.